If McVitie’s had trademarked its Jaffa Cakes some time after inventing them in 1927, we might not eat as many as we do. But the generic cake is now the favourite of MPs and House of Commons staff, and quite possibly of the country as a whole, since McVitie’s alone bakes more than a billion of them a year. The Jaffa is, of course, a cake and not a biscuit, as every VAT expert knows, because a biscuit turns limp when stale, whereas a cake hardens. MPs like biscuits too, of course, but when confronted with a choice of Jaffas and digestives, they are 12 times more likely to choose the cake. Small children, and possibly some adults, like to eat the sponge and chocolate first, before sucking the shimmering orange disc. But dunking a Jaffa remains the ultimate test of hand-eye coordination: immerse it a microsecond too long and the whole thing vanishes.